1. Field of the Invention
The present invention relates to a method of manufacturing a surface mounted (SMD) piezoelectric vibrator where a piezoelectric vibrating reed is sealed in a cavity formed between two bonded substrates, a piezoelectric vibrator manufactured by the method, and an oscillator, an electronic device, and a radio clock that each include the piezoelectric vibrator.
2. Description of the Related Art
In recent years, a piezoelectric vibrator, which employs crystal or the like as a time source, a timing source of a control signal or the like, a reference signal source or the like, has been used in a cell phone or a portable information terminal device. Various piezoelectric vibrators have been known as this kind of piezoelectric vibrator, but a surface mounted piezoelectric vibrator has been known as one of the piezoelectric vibrators. As this kind of piezoelectric vibrator, there is generally known a piezoelectric vibrator having a three-layer structure where a piezoelectric substrate including a piezoelectric vibrating reed is bonded to a base substrate and a lid substrate so as to be interposed between the base substrate and the lid substrate in a vertical direction. In this case, the piezoelectric vibrating reed is received in a cavity (closed chamber) that is formed between the base substrate and the lid substrate.
Further, in recent years, there has been developed a piezoelectric vibrator having not a three-layer structure but a two-layer structure. This type of piezoelectric vibrator has a two-layer structure where a base substrate and a lid substrate are directly bonded to each other and a piezoelectric vibrating reed is received in a cavity formed between both the substrates. The two-layer structure type piezoelectric vibrators are superior to three-layer structure type piezoelectric vibrators in terms of the reduction in thickness. Accordingly, the two-layer structure type piezoelectric vibrators are preferably used.
Meanwhile, when the two-layer structure type piezoelectric vibrator is manufactured, the base substrate and the lid substrate are anodically bonded to each other by using a bonding film formed on the lower surface (bonding surface) of the lid substrate. Specifically, after being superimposed, both the substrates are set on an electrode plate of an anodic bonding apparatus. Subsequently, while the base substrate is heated, a voltage is applied between the bonding film and the electrode plate. In this case, since the base substrate is heated, ions in the base substrate have flowability. Accordingly, if a voltage is applied between the bonding film and the electrode plate simultaneously with this, current flows in the base substrate. Therefore, it may be possible to make an electrochemical reaction occur on the interface between the bonding film and the base substrate, and to make the bonding film and the base substrate come into close contact with each other and be anodically bonded to each other.
Meanwhile, as this type of piezoelectric vibrator, there is known a piezoelectric vibrator where a bonding film is formed not only on the bonding surface but also on the entire lower surface of a lid substrate (for example, Patent Citation 1). One example of this piezoelectric vibrator will be briefly described with reference to drawings. As shown in FIGS. 18 and 19, a piezoelectric vibrator 200 includes a base substrate 201 and a lid substrate 202 that are bonded to each other, and a piezoelectric vibrating reed 203 that is sealed in a cavity C formed between both the substrates 201 and 202. The piezoelectric vibrating reed 203 is, for example, a tuning-fork type vibrating reed, and is mounted on the upper surface of the base substrate 201 in the cavity C.
Each of the base substrate 201 and the lid substrate 202 is, for example, a glass substrate. Through holes 204, which pass through the substrate 201, are formed at the base substrate 201 of both the substrates 201 and 202. Further, a conductive member is fitted to each of the through holes 204 so as to close the through hole 204, so that a through electrode 205 is formed. The through electrodes 205 are electrically connected to an external electrode 206 formed on the lower surface of the base substrate 201, and are electrically connected to the piezoelectric vibrating reed 203 mounted in the cavity C. Further, a bonding film 207 is formed on the entire lower surface of the lid substrate 202, and both the substrates 201 and 202 are anodically bonded to each other by using the bonding film 207.    Patent Citation 1: JP-A-6-283951
Meanwhile, there have been the following problems in the anodic bonding used to manufacture the piezoelectric vibrator 200 where the bonding film 207 is formed on the entire lower surface of the lid substrate 202. Specifically, first, the base substrate 201 and the lid substrate 202, which are superimposed, are set on an electrode plate 208 as shown in FIG. 20 at the time of the anodic bonding. Then, a bonding voltage (for example, 0.5 to 5.0 kV) needs to be applied between the bonding film 207 and the electrode plate 208 while all of them are heated to a bonding temperature (for example, 100 to 200° C.).
In this case, the through electrodes 205, which are formed at the base substrate 201, come into direct contact with the electrode plate 208. Accordingly, when the bonding voltage is applied between the bonding film 207 and the electrode plate 208, substantially the same voltage as the bonding voltage might be applied even between the bonding film 207 and the through electrode 205. In particular, the through electrodes 205 are electrically connected to the piezoelectric vibrating reed 203 that is close to the bonding film 207. For this reason, there has been a concern that electric discharge (spark discharge) occurs from the bonding film 207 toward the through electrodes 205 through the piezoelectric vibrating reed 203.
As a result, there has been a problem in that the through electrode 205 having been subjected to electric discharge is melted. For this reason, there has been a possibility that electric conductivity between the piezoelectric vibrating reed 203 and the external electrode 206 becomes unreliable. In addition, if the through electrode 205 is melted, it may not be possible to seal the through hole 204. For this reason, there also has been a possibility that airtightness in the cavity C cannot be secured.
Further, if electric discharge occurs, sufficient current does not flow in the bonding film 207. For this reason, there has been a problem in that anodic bonding between the base substrate 201 and the lid substrate 202 is affected. Therefore, it was not possible to reliably seal both the substrates, and it also was difficult to secure airtightness in the cavity C.